<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        nav{
            height: 80px;
            background-color: red;
        }
        .box1{
            width: 400px;
            height: 400px;
            position: fixed;
            top: 50%;
            left: 50%;
            transform: translate(-50%,-50%);
        }
        .box1_1{
            height: 20%;
            background-color: green;
        }
        .box1_2{
            height: 80%;
            background-color: yellow;
        }
        .mask{ 
            width: 100%;
            height: 100%;
            background-color:rgba(0,0,0,.3);
            position: fixed;
            top: 0;
            left: 0;
        }
    </style>
</head>
<body>
    <div class="mask"></div>
    <nav>导航</nav>
    <div class="box1">
        <div class="box1_1">标题</div>
        <div class="box1_2"></div>
    </div>
    <img src="../img/footer.jpg" style="margin-top: 1000px;">
</body>
</html>